Technical Abstract:
Cold temperatures often restrict early vegetative growth of maize (Zea mays L.) grown in temperate climate regions. Photosynthesis and root growth are particularly temperature sensitive. Reduced growth under low temperatures could cause a carbohydrate source:sink imbalance leading to feedback inhibition of photosynthesis. In this study, two maize hybrids differing in their growth sensitivity to cold were sown in replicated field plots in early and late spring in west central Minnesota, USA. Photosynthesis at the two-leaf stage for the early-sown cold-tolerant (CT) hybrid was 26% greater than the cold-sensitive (CS) hybrid, though PSII maximum quantum yield (FV/FM) was only 7% greater. There was no difference in photosynthesis between hybrids from the later sowing date. For early-planted CT maize, total PEPC and Rubisco activity were 28 and 35% greater than the CS hybrid, respectively. Both PEPC and Rubisco protein content followed a similar trend. Diurnal soluble carbohydrate content in the second leaves of both early-sown hybrids did not differ greatly. However, nonstructural carbohydrate content, especially sucrose and hexoses, were considerably greater in roots of CT as compared to CS maize. Results indicate genetic differences exist in maize for acclimation of carboxylation capacity to low temperatures and that this may be driven by sink strength, particularly in roots during early vegetative growth. Further work is being done to determine the effects of cold on PEPC and Rubisco gene expression in CT and CS maize.